I guess this is true of all formal languages, then. Since we sit in front of text editors most of the time, the fundamental truth of all languages is that they are strings.
This is not even true of natural language, which has a vocal representation that is at least as important as the written representation.
Though I agree that representing HTML as objects is a poor substitute.
It's a bit deeper: since HTML is defined as text markup, text is the truth of HTML documents and the standard of their users, while any sort of object representation of HTML documents is someone's idiosyncratic and possibly limited implementation, necessarily specialized and necessarily harder to use.
On the contrary - the entire purpose of HTML is to construct a specific object model in a web browser. HTML is a serialization format for expressing DOM structure. It’s not ‘idiosyncratic’, it’s the way the language is defined.
This is not even true of natural language, which has a vocal representation that is at least as important as the written representation.
Though I agree that representing HTML as objects is a poor substitute.